To

#3, I suppose.

Not #1?

2. is not an infinitive construction, really. This is equivalent to "He must go there alone.", the combination "is to" idiomatically meaning "must."

In 1. the infinitive construction functions as a noun meaning the same thing as "The job of a politician."

3. is not an infinitive construction, really. This is equivalent to "This letter will inform you that we have received your

Anonymous"3. is not an infinitive construction, really. This is equivalent to "This letter will inform you that we have received your request.", the combination "is to" idiomatically meaning "will."

I wonder if such replacement always works.

As with many other areas of alternative medicine, controlled clinical trials are scarce; but this is

"This is not to say that none at all are available." is not really an infinitive construction. "This is not to say that" is an idiomatic expression meaning "This does not mean that." So the sentence means: "This does not mean that none at all are available."

Another way of looking at sentence 3. is that it is equivalent to: "This letter is for the purpose of informing you that we have received your request." And as such, we have an adjectival structure linked to the subject by the linking verb "is," which modifies the subject.

Then the fourth sentence would be equivalent to: "None of this is for the purpose of denying that he is innocent."